After completing Lifting the Vale one can pick the lock on the display case in the throne room to retrieve the Madstone. It will be counted as stolen however so one will need to sell it to a Thieves' Guild fence then buy it back in order for it to not be taken in case they are arrested.

Countess Carvain tells them at the start of the quest that the Madstone protects its wearer from all poisons, however the amulet only has a 50% enchantment.

This could be a reference to the madness of Emperor Draco of Ancient Greece.